Sprayed concrete tunnelling technology has progressed rapidly over the last five years. Dave Parker reports on some key innovations.
Adecade or more ago, sprayed concrete meant 'gunite'. Compressed air delivered a dry mixture of sand and cement to a special nozzle, where the operator judged how much water to add. Without further mixing the concrete was then blasted onto a solid surface - from which up to 40% promptly rebounded.
Provided the operator was highly skilled and prepared to tolerate the dusty environment in the tunnel, the finished product was deemed suitable for 'temporary support', as in the New Austrian Tunnelling Method. But poor mixing and operator sensitivity meant that classic dry-mix sprayed concrete was too variable for most structural uses.
Wet mixing, however, was far from a straightforward alternative. True, the finished product would be properly mixed and far more predictable; it would be stronger, and have far less rebound, rarely as much as 10%.
But the maximum distance from mixer to nozzle was limited to about 300m, and any breakdown could see concrete hardening in the delivery lines. With most UK specialists wedded to the drymix process, virtually all sprayed concrete in UK tunnels was dry-mixed - until the last decade of the 20th Century.
Then, a combination of the failure of a dry-mixed temporary lining on the Heathrow Express project and the success of a wetmixed contract on the Jubilee Line Extension began to change minds. Latest figures show that around 90% of the more than 50,000m 3of sprayed concrete applied in the UK every year is wet mixed, a similar proportion to France and Japan.
There are two main reasons for this swing, according to Ken Dykes, business development manager at specialist contractor Makers (UK): 'Much-improved admixtures, especially alkalifree accelerators. And robotic spraying systems. These have come on by leaps and bounds, and can spray up to 20m 3/hour, compared to more like 6m 3/hour with the old dry-mix system.'
A whole family of advanced admixtures has been developed to smooth out the wet-mix system glitches. Short 'shelf life' - wet mixes had to be applied within a maximum of two hours - was extended by the use of hydration controllers like MBT's Delvocrete Stabiliser. Added in the mixer, these coat the cement grains and stop them reacting with the water for as long as 72 hours. Eight hours is more typical. Then, when spraying begins, another admixture is added at the nozzle to restart hydration.
Higher workability mixes are easier to mix and pump but would be likely to slump and debond after spraying. Adding a high performance set accelerator at the nozzle rapidly stiffens the mix, even in mid-air between the nozzle and the surface.
Alkali-free accelerators, although much more expensive than the silicate or aluminate-based alternatives, have the advantages of environmental friendliness and safety of handling as well as reduced risk of triggering alkaliaggregate reactions in the finished concrete.
Such advantages come at the price of reduced 28-day compressive strengths. Old style aluminate accelerators could cut these by 50%. But, by adding one of the new generations of super or hyper plasticisers, and reducing water/cement ratios to as low as 0.3, it is still possible to produce high strength accelerated concretes with excellent spraying performance.
Adding up to 10% microsilica will cut rebound even further.
New high performance polyethylene fibres look like taking over from steel fibres, and eliminate the installation of steel mesh reinforcement completely.
The result of all these developments, says admixture specialist Feb MBT development manager Ian Wilson, is that the first sprayed concrete lining can now be considered to be a permanent long term structural element.
'On the Channel Tunnel Rail Link's North Downs Tunnel project the primary sprayed concrete lining was treated as 'grey rock', enabling the secondary lining to be redesigned for much better 'ground conditions'.
'Overall the lining thickness was reduced, reinforcement to the insitu secondary lining eliminated - and £10M was saved. That's 13% of the contract value.'
Remotely controlled robotic spraying equipment was used to achieve high output rates. Modern piston type pumps can almost eliminate the pulsation that makes it difficult to achieve constant thickness, while automatic admixture dosage takes out the guesswork.
A package like this makes allsprayed linings a feasible alternative. A further development could be decisive. Sprayable waterproof membranes which also help bond inner and outer layers of sprayed concrete together are a much superior alternative to sheet membranes, and can resist up to 15 bar of water pressure. Feb MBT's Masterseal 340F also has the advantage of being waterbased, making it easier to use in confined spaces.
Ultimately, of course, advanced technology can never entirely replace human skills. A key development has been the introduction of the Sprayed Concrete Association's certification scheme for nozzle operators, which covers both dry and wetmixed operatives. Dry mix sprayed concrete will still be used but mainly to repair old railway tunnels.